1. Field of the Invention
The present invention relates to a drive circuit for driving a magnetron equipped in a microwave oven, and more particularly, to a microwave oven driving circuit by which the magnetron output can be suitably generated when the output level of the microwave oven must be changed. The driving circuit includes a microcomputer for controlling the output level of the microwave oven at the initial operation or in a cooking process.
2. Description of the Prior Art
In general, when a power switch is turned on to operate a microwave oven, an amount of hot electrons emitted from a filament of a magnetron is very low at an initial operation, when the filament is insufficiently preheated. As a result, the normal magnetron output cannot be generated, thus resulting in a disadvantage that a secondary coil of a transformer is not loaded while a high voltage is supplied to a primary coil of the transformer.
To prevent the aforementioned situation, a conventional method involves controlling the magnetron to generate a very low output so as to not supply high voltage to the primary coil of the transformer until the normal magnetron output can be generated.
The result of the conventional method described above is that a minute current flows to the primary coil of the transformer so that the magnetron is supplied with a voltage less than an operational voltage, i.e., a threshold voltage thereof. Consequently, a problem arises in that the generating operation of the magnetron cannot be appropriately performed.
The problem is more serious where the magnetron is supplied with a voltage less than the rated voltage thereof due to a change of common power supply.
Also, during an operation of the microwave oven, when the output of the magnetron is to be changed from a high level to a low level according to a cooking program previously stored in the microwave oven (that is, when the cooking is nearly completed and a high output from the magnetron is unnecessary), a moding, phenomenon wherein the magnetron output is not generated, instantaneously occurs from a property of the magnetron, thereby deteriorating a function of the magnetron.
To solve such a problem, there is well-known a cooking method described in, for example, Japanese Patent Laid-Open Publication No. Sho 61-296678, by which a magnetron is continuously driven and an electric heater is simultaneously operated.
More particularly, a circuit arrangement of the cooking method disclosed in the Japanese patent publication, includes a rectifying circuit connected to an ac power supply; a transformer connected at its primary winding to an output terminal of the rectifying circuit a magnetron connected to a secondary winding of the transformer through a voltage multiplier comprising a capacitor and a diode; a resonance capacitor forming a resonance circuit together with the transformer; a first switching element for excitating the resonance circuit an electric heater connected to an output terminal of the rectifying circuit through a second switching element and, a control means for turning ON or OFF the first and second switching elements with a predetermined duty ratio so that the sum of the output of the electric heater and the output of the magnetron is at a predetermined value, thereby obtaining a good efficiency in heating the magnetron.
The conventional cooking method described above has the advantage that the magnetron is continuously driven and a filament is preheated by means of the electric heater, but the method has disadvantages in that a structure is complicated and power consumption is necessarily increased because of the electric heater.